1. Field of the Invention
The present invention relates generally to a cutting tool, and more particularly to a cutting bit insert and ring of the type for fixing to a rotable shank where the bit insert is a polygonal pyramid and the bit insert is harder than the ring.
2. Description of the Related Art
Various types of configurations have been used in the past for the bits of abrading or cutting instruments. It is conventional in the art to attach a cutting bit insert made of a blended material, which includes some tungsten carbide, to one end of a shank and to then insert the shank into a block. The block is then pushed along a surface, the force of the surface on the cutting bit insert causing rotation of the bit insert and shank and abrasion or cutting of the surface. The use of this general configuration is old in the art.
Many of the modifications of this general configuration have been made to the cutting bit insert and to the geometry of the juncture between the bit insert and the shank. Examples of these types of modifications can be found in the patents to Den Besten et al., U.S. Pat. No. 4,201,421; Ojanen, U.S. Pat. Nos. 4,497,520 and 4,547,020; Penkunas et al., U.S. Pat. No. 4,725,099; Mills, U.S. Pat. Nos. 4,823,454 and 4,932,723; Stiffler et al., U.S. Pat. Nos. 4,911,503 and 4,940,288; Larsson et al., U.S. Pat. Nos. 4,938,538 and 5,161,859; and Stiffler, U.S. Pat. No. 4,941,711.
Other modifications have been made to the cutting bit insert itself by including radially extending ribs along a portion of the cutting bit insert. Examples of this type of modification can be found in the patents to Maddock, U.S. Pat. No. 3,361,481; Radd, U.S. Pat. No. 3,746,396; Rowlett et al., U.S. Pat. No. 5,131,725; Massa et al., U.S. Pat. No. 5,324,098; and Sollami, U.S. Pat. Nos. 5,484,191 and 5,551,760.
Other modifications have been made to the shank, the block, or the joints between the cutting bit insert and the shank or the shank and the block. These modifications have been designed to increase the durability of the various parts and keep them from deteriorating over time from abrasive contact with the asphalt. Examples of structures which attempt to increase durability are shown in the patents to Dziak, U.S. Pat. No. 4,489,986; Beebe, U.S. Pat. No. 4,561,698; Mills, U.S. Pat. No. 4,660,890; Beach, U.S. Pat. No. 4,725,098, and Graham et al., U.S. Pat. No. 5,417,475.
However, even though there have been numerous designs created and previously used, it is desirable that an improved configuration be used which further increases the abrasion properties of the bit while decreasing the erosion of the remainder of the structure. The present invention is an improvement in this area and includes these and other important features.